1. Field of the Invention
This invention relates generally to loudspeaker systems and, more particularly, to a sound system in which the audio frequency is divided into upper and lower ranges. The instant invention concerns itself with the particular location of a high-frequency (HF) transducer with respect to certain elements of a low-frequency (LF) transducer, and the particular means for attachment of various elements of the high-frequency transducer to various moving (dynamic) elements of the low-frequency transducer. The collocation of both high-and low-frequency transducers on the same axis is referred to herein as coaxial mounting.
2. Prior Art Discussion
The goal in loud speaker design is to create a device which will accurately transform an electrical signal wave form voltage into an exact replica of the original acoustic pressure wave form which created it. The typical music wave form is a complicated combination of continuous tones, overtones and transient sounds. Also, several instruments or voices are usually superimposed and thus, the total wave form can be thought of as the summation of all the different frequencies which make up the sound as perceived by the listener. The primary characteristics of sound wave forms are phase, amplitude and frequency. The first, phase, refers to the timing relationship of the various frequencies and the transients as they reach the listener's ear. Amplitude is relative size, magnitude or intensity of the various frequencies and transients. Frequency refers to the number of cycles per second of the audio signal, (termed) its pitch. It is possible to identify each instrument or voice of a sound replication by its distinctive combination of overtones and by its location in the stereophonic image. Because of such physiological characteristics as persistence, the human ear is fooled into a "recognition" of the original sound. The ideal speaker device (also referred to herein as a "transducer") is one which is perfectly faithful in phase, amplitude and frequency response--high fidelity, or high "faithfulness".
In the beginning of high fidelity sound replication, the most prevalent form of loud speaker, or electro-audio transducer, was the dynamic radiator speaker. This device essentially comprised a permanent magnet with an air gap and a concentric pole piece on which was mounted a hollow, nonmagnetic, conducting shell known as a former and which was wrapped with an insulated wire termed a voice coil. The former was directly coupled at its outer margin to the apex or minor perimetral margin of a conical paper shell, known as a diaphragm and which projected outward of the permanent magnet assembly. As the voice coil was excited within its permanent magnet environment, the voice coil moved longitudinally and the diaphragm was caused to vibrate, thus replicating the audio (air pressure dynamic) signal that had generated the electrical current passing through the voice coil by pressing dynamically on the contact air mass in front of the diaphragm. The dynamic radiator had several characteristics that were noteworthy, and necessary, to the high fidelity replication of sound at that time: it was relatively efficient; it had a fairly wide frequency range; and it could be used as a low-range, mid-range or high-range electro-audio transducer. Concomitantly, and more so in present context, the dynamic radiator has notable limitations: a single driver cannot cover the entire audible frequency range of 20 hz. to 20 KHZ; multiple drivers (coils) must be used for different parts of the audio spectrum; cross over circuits must be used to separate the audio spectrum; and oftentimes, heavy magnets and rigid support frames are necessary to construct the assembly.
The quest for high fidelity sound reproduction which began in the early 40's, has continued unabated to the present. Of significant relevance to the instant invention is U.S. Pat. No. 2,269,284, issued to Olson in 1942. This patent, referred to in a host of applications since that time, has set the stage for much of the sound reproduction devices relating to high fidelity and stereophonic sound. Olson taught, basically, the coaxial arrangement of multiple speakers, one within the other, and aligned along a central, common core. In the Olson art, the diaphragms of the various loud speakers are generally conically shaped and are arranged in nested, overlapping relationship with their cylindrical driving coils that are arranged concentrically in radially spaced relationship in the air gap formed between the pole pieces of a suitable magnetic structure. The base conical section, or low range speaker (hereinafter referred to as a "woofer") is the first of the loud speakers to be mounted, relative the pole piece. Thereafter, succeedingly smaller conical sections of the relatively smaller loud speakers are nested one within the other, digressing in size to the physically smallest, highest frequency range electro-audio transducer (hereinafter referred to as "tweeter"), with all their diaphragms and respective driving coils forward of the base woofer assembly. Many modifications of the basic invention are referred to in the Olson patent, but it may be generally summarized as a system having the equivalent of large cones and coils for the low audio frequencies, medium size cones and coils for mid-range sound reproduction, and small cones and coils for high-frequency reproduction. The voice coils are generally connected in series with a predetermined capacitance connected across each of the specific coils. With such an arrangement, at low frequencies, a signal flows through all three coils. Concurrently, the reactance and the compliances are small compared to the mass reactance of the several coils; thus, all parts of the system move in (with the same) phase. In the mid-range, very little current appears in the woofer coil because of the effective shunt provided by its capacitance. The mass reactance of the coil is large compared to the compliance; therefore, at such ranges, the diaphragms of both the mid-range and high-range cones are driven by their respective coils. At the high frequencies, the compliance between the tweeter coil and the mid-range coil is small compared to the mass reactance of the mid-range coil, and practically all the current appears in the tweeter coil because of the shunting effected by the woofer and mid-range capacitors, causing the tweeter driver to vibrate its respective cone and produce the desired audio radiation at the higher frequencies. It may be said generally that, since Olson, most of the significant advancements have been made in the placement of the various mid-range and high-range (tweeter) transducers, as well as in the use of different driving mechanisms, such as piezo-electric, electrostatic, magnaplanar, ribbon, plasma, etc. Of the many types of current speaker design (driver mechanisms), the most common and least expensive are the dynamic radiator and piezo-electric. The instant invention contemplates exclusive use of these two types of speaker design.
In 1947, Preston, in U.S. Pat. No. 2,426,948, disclosed a Coaxial Dual-Unit Electrodynamic Loud Speaker in which the tweeter unit was electrically crossed and capacitively coupled to the woofer unit, and coaxially mounted, so that its permanent magnet supporting base resided within the woofer central pole piece. As with the Olson art, however, Preston continued to arrange the driving coils concentrically and in basically the same transverse plane in which the woofer voice coil resided. A compliant member was used to essentially join the tweeter diaphragm to that of the woofer. Compliances are generally used for joining purposes.
In 1985, two patents issued to House, U.S. Pat. No. 4,497,981 and U.S. Pat. No. 4,554,414 for a Multi-Driver Loudspeaker. In the first, '981, a multi-driver loud speaker assembly having high- and low-frequency transducers is realized in which the high frequency transducer is directly coupled to the diaphragm of the low frequency transducer and is movable therewith. In the later patent, '414, a multi-driver loud speaker combination includes a first transducer of the dynamic radiator type (previously discussed), which is designed to reproduce sound in the lower portion of the audio frequency range. The radiator of the first transducer includes a diaphragm and, concentrically aligned and coaxially mounted therein is the second transducer, or tweeter assembly. A horn shaped base support is mounted on the first transducer diaphragm, a voice coil former, or the dust cap which is generally employed with singular mechanizations of the dynamic radiator type transducers. In this arrangement, House suggests more than one type of orientation of the tweeter assembly with respect to the woofer. Significantly different from the previously mentioned prior art is the utilization by House of a piezo-electric transducer for driving the tweeter assembly. Later, in this disclosure references will be made to the general art disclosed by House; and, the piezo-electric tweeter driver shall be referred to more casually as "bi-morph" element. The House art, clearly relevant art at this late date, will be discussed in greater detail and in contrast to the techniques embodied by the instant inventor in realizing this improved, dynamic, bi-morph speaker.
Since the piezo-electric bi-morph element comprises a prominent part of the instant invention, a few words descriptive of its structure are in order. By definition, the driving element of the tweeter loud speaker is known colloquially as a "bi-morph", i.e., a bi-layer, amorphic ceramic element. The bi-morph wafer is composed, essentially, of two ceramic plates in the form of discs, with a voltage conductor (plane) sandwiched therebetween. When a driving electromotive force is applied to both of the ceramic plates and at the conducting voltage plane, piezo-electric effect causes flexure of the element in a direction normal to the plane of the element plates. The apex of the tweeter diaphragm cone is affixed proximate the center (and maximum flexure point) of the bi-morph wafer. Thus, the diaphragm of the tweeter translates the dynamics of the bi-morph element radially outward from its apex towards its largest or major perimetral margin. Conventionally (in the current art), the larger perimetral margin of the tweeter, or any mid-range speaker, is joined to the diaphragm of another speaker by some compliant material (compliance).
The instant inventor believes that because Olson and Preston did not have the advantage of the bi-morph element, they were not impelled to greater innovation in the coaxial mounting scheme that they pioneered. Because House was concerned with the structural dynamics of plural tweeters or mid-range speakers, he strayed somewhat from the pure coaxial mounting scheme and thus failed to give greater definition to an allusion of mounting the bi-morph element on (to) the woofer voice coil former.
The final piece of relevant art to be discussed is Japanese Application No. 57-122303, laid open 59-12700(A), issued to Ishikawa for a Composite Type Speaker. It was Ishikawa's purpose to obtain a composite type speaker of low cost and high efficiency by providing a piezo element, constituting a tweeter, and position it at a prolonged part of a woofer voice coil former. Ishikawa allowed the bi-morph element to merely touch the rim of a voice coil former extension while coupling the edge or rim of the element to the woofer cone via a paper extension which served as a (form of) cone for the tweeter. Illustrations in the Ishikawa disclosure, notably FIGS. 5 and 7, clearly indicate that the piezo element was not rigidly or fixedly secured to the voice coil former, nor any portion thereof. Thus, Ishikawa speaks of "coupling" rather than fixedly attaching (or securing) the piezo element to the voice coil former. In contrast, it shall be seen hereinafter that the instant inventor expressly attaches at least a portion of the piezo element margin (or periphery) to a dynamic part of the woofer. Where Ishikawa attempts to show that the posturing of the piezo element so that it touches the rim of the former extension, and suffers no ill effect thereby, the instant inventor contrarily secures it thereto and literally invites, rather than seeks to avoid, concurrent movement of both piezo element and woofer parts.
The instant inventor, wishing to develop a more highly efficient and higher fidelity coaxial (general, but not insistent) arrangement was inspired to perform a wide variety of experiments with placement and attachments of the bi-morph driven tweeter. The instant improvement is the result of his efforts.
Hereinafter the instant invention will be briefly disclosed, and in the following Detailed Description of the Preferred Embodiment, a comparison to the prior art of House and (later) Ishikawa shall be made; and, the benefits and improvements of the instant invention shall be clearly discussed and detailed.